The balance sensing and spatial orientation functionality of the brain is developed based on neural signals from the vestibular structures of the inner ear, one on each lateral side of the body. As shown in FIG. 1, each inner ear vestibular labyrinth 100 has five sensing organs: the ampullae 108 of the three semi-circular canals—the posterior canal 103, the superior canal 104, and the horizontal (lateral) canal 105—which sense rotational movement, and the utricle 106 and the saccule 107 in the vestibule 102, which sense linear movement.
FIG. 2 shows anatomical detail within a vestibular canal ampulla 108 which is connected at one end to the canal 206 and at the other end to the vestibule 205, and which contains endolymph fluid. The vestibular nerve endings 204 connect to the crista hair cells 203, the cilia ends 202 of which are embedded in the gelatinous cupula 201. When the head is stationary, the vestibular nerve endings 204 generate a baseline level of neural activity that is transmitted to the brain. When the head moves, the endolymph fluid within the respective ampulla 108 defects the cupula 201, that changes the neural activity level at the corresponding vestibular nerve endings 204 that correlates with the direction of head movement.
Unfortunately some people suffer from damaged or impaired vestibular systems in which the brain receives no inputs or meaningless inputs from the vestibular system. Such vestibular dysfunction can cause balance problems such as unsteadiness, vertigo and unsteady vision. Such sufferers lack the ability to balance and orientate, and instead have to rely on vision and proprioceptive inputs for balance.
The patient may also lose the vestibulo-ocular reflex (VOR), which allows for quick eye movements to compensate for head movement when focusing on a target. Without this VOR, the eyes cannot focus on the target during head movement (oscillopsia). With unilateral vestibular loss, the contralateral (non-diseased) inner ear is able to compensate and provide enough balance information. However, in the case of bilateral loss of vestibular function, the body can adapt to some extent by replacing the VOR reflex with the cervico-ocular reflex (COR), which sends signals from the neck to the eyes, saccades to keep the eyes focused on the target, or to implement strategies for eye movement based on anticipation or prediction. Despite this ability, in many patients vestibular function is not significantly restored and a vestibular prosthesis is needed.
Vestibular prosthesis systems are currently being researched that deliver electrical stimulation to the vestibular system to restore vestibular function to those who suffer from vestibular related pathologies. A vestibular prosthesis system needs to measure head movements and provide corresponding electrical stimulation patterns to the respective branches of the vestibular nerve. The prosthesis should be selective to avoid cross-talk and unintended stimulation of non-target vestibular nerve branches, and also to avoid stimulating other neighboring anatomical structures such as the facial nerve and the cochlear nerve. It is important for the prosthesis to have an atraumatic design that preserves hearing and any residual vestibular function, and so also preserves possibility of future alternative treatments. To preserve hearing, it is important to avoid penetration of the membranous labyrinth of the vestibular ampulla to preserve the volume of endolymph therein and maintain the natural ionic properties of the endolymph.
Experimental results indicate that electrical stimulation of the vestibular system has the potential to restore vestibular function, at least partially. See, e.g., Rubinstein J T et al., Implantation of the Semicircular Canals With Preservation of Hearing and Rotational Sensitivity: A Vestibular Neurostimulator Suitable for Clinical Research, Otology & Neurology 2012; 33:789-796 (hereinafter “Rubinstein”); Chiang B et al., Design and Performance of a Multichannel Vestibular Prosthesis That Restores Semicircular Canal Sensation in Rhesus Monkey; IEEE Trans. Neural Systems and Rehab Engineering 2011; 19 (5):588-98 (hereinafter “Della Santina”); and Gong W et al., Vestibulo-Ocular Responses Evoked Via Bilateral Electrical Stimulation of the Lateral Semicircular Canals, IEEE Transactions On Biomedical Engineering, Vol. 55, No. 11, November 2008 (hereinafter “Merfeld”); all incorporated herein by reference.
One challenge in developing a vestibular implant is the design of a device-to-body interface, the stimulation electrode. Such a vestibular stimulation electrode is inserted into the vestibular canal to selectively stimulate at least one of the vestibular nerve branches for the vestibular canal ampullae. Typically insertion of the stimulation electrode is though the semicircular canal. The stimulation electrode should be located as close as possible to the nerve fibers of the hair cells in the ampulla crista without damaging them.
Currently, different research groups are working on the development of different vestibular implants, with intra-labyrinthine stimulation approaches being of interest for the present purposes. U.S. Pat. No. 7,962,217 of the Merfeld group addresses Meniere's disease and is not intended for selective stimulation as required for treatment of vestibular disease. The Merfeld group also has published information on use different types of stimulation electrode including simple wires (Gong et al., Vestibulo-Ocular Responses Evoked Via Bilateral Electrical Stimulation of the Lateral Semicircular Canals, IEEE Transactions On Biomedical Engineering, Vol. 55, No. 11, November 2008), and polyimide thin film electrodes (Hoffman et al., Design of Microelectrodes for a Vestibular Prosthesis, BMT 2011 Rostock, Germany), though for the latter there is no published data.
The Rubinstein research group published details of a vestibular stimulation electrode in the previously cited Rubinstein article, as well as in U.S. Patent Publication 2012130465 and U.S. Patent Publication 2012/0226187. Their stimulation electrode has a relatively small diameter to prevent compression of the membranous canals using “soft surgery” techniques. They claim to have developed a vestibular stimulation electrode that allows post-surgical preservation of the natural function of the vestibular system.
The Della Santina research group published details of their stimulation electrode in the previously cited Chiang reference, as well as in U.S. Pat. No. 7,647,120 and PCT Patent Publication WO 2011088130. Their prosthesis is being developed for treatment of bilateral vestibular hypofunction (BVH) for which there is no absolute need to preserve natural vestibular function. The research and development strategy here accepts compression or other trauma to the membranous labyrinth in order to get the stimulation electrodes closer to the respective nerve branches. Since the membranous duct fills out almost the entire ampulla, it is virtually impossible to reach the crista without compressing or otherwise traumatizing the membranous canals.
All the above mentioned examples are electrodes that are intended to be inserted into the vestibular labyrinth, known as intra-labyrinthine electrodes. There are also extra-labyrinthine electrodes that are placed outside the vestibular labyrinth to stimulate the vestibular nerve. There are researchers that are attempting to locate such electrodes in close proximity to individual ampullary nerve branches of the vestibular nerve. The main advantages of using extra-labyrinthine electrodes is the preservation of the delicate intra-labyrinthine structures (which reduces the risk of generating a sensorineural hearing loss) and the closer distance to the addressed nerve branches.
The main disadvantages of extra-labyrinthine electrodes relate to the surgical accessibility of the ampullary nerve branches. When drilling in close proximity to nerves, there is increased risk of damaging the nerve. And to approach the lateral and superior ampullary nerve branches, parts of the ossicular chain need to be removed, which results in a conductive hearing loss. The lateral and superior ampullary nerve branches also are in close proximity to the facial nerve, which increases the risk of damaging the facial nerve and/or unintentionally stimulating it. So, even though extra-labyrinthine electrodes may preserve the intra-labyrinthine vestibular structures, they are traumatic for other anatomical structures and/or increase the risk of mechanically damaging the nerve during surgery.